Exploration for oil and gas requires testing of the earth's substructure by seismic techniques. Seismic sources of great variety have been used in the past. For instance, the explosion of a small charge buried 10 or 20 feet deep has been used for many years in the seismic industry. Explosives, of course, are dangerous in handling and their use has in large extent been supplanted by other techniques. Other techniques include the dropping of a large weight on the surface. Freefall weights were proposed many years ago in the patents of Burton McCollum. The use of an explosive mixture of air and combustible gases filling an inverted dome has likewise been used. Another technique is the generation of a frequency sweep where a loud speaker is effectively coupled with the soil. The Vibroseis of the Continental Oil Company is representative of this type of equipment.
The apparatus of the present invention is believed to provide a substantial improvement over seismic sources mentioned above. The apparatus of the present invention is able to obtain subsurface coupling similar to that of explosive charges. However, it does not have the danger or problems of handling which are normally associated with explosives. It is more rapid in use and operation because it can be triggered repetitively. By contrast, explosive charges must be positioned and ignited which is a sequency of events requiring some time. The apparatus of the present invention is particularly adapted for use in areas of changeable topography. It is adapted to be used in marshes and swamps where seismic shock point will be on dry land, the next under water, the next in sand, and so on.
As background for the seismic source of the present invention, references is made to U.S. Pat. No. 3,639,752 which discloses several forms of a seismic source. The patent discloses what will be described as a pneumatic sonic source. In general, the patent discloses the use of a chamber which accumulates air under very high pressure. An external port or window is closed by a valve member. The valve is triggered for a short interval, exposing the port, and permitting air to escape through the port. The valve is subsequently closed, interrupting flow from the port. The air rushes through the window or port creating noise as it flows, and forming a shock wave. It typically contemplates pressures as high as 5000 or 6000 psi with a drop to about 2000 psi on discharge. The apparatus can be rapidly recharged through the use of an air compressor to create another shock wave.